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प्रश्न
A nearsighted person cannot see beyond 25 cm. Assuming that the separation of the glass from the eye is 1 cm, find the power of lens needed to see distant objects.
उत्तर
Given:
As the person wears spectacles at a distance of 1 cm from the eyes.
Distance of the image from the glass, v = Distance of the image from the eye − Separation between the glass and the eye
v = 25 − 1 = 24 cm = 0.24 m
For the near-sighted person, u = ∞.
The lens formula is given by
`1/v -1/u = 1/f`
On substituting the values in the above formula, we get:
`1/f = 1/((-0.24))-1/∞ = -42`
Power of the lens, P = `1/f` = − 4.2 D
Thus, the power of the lens required to see distant objects is − 4.2 D.
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संबंधित प्रश्न
For a normal eye, the far point is at infinity and the near point of distinct vision is about 25cm in front of the eye. The cornea of the eye provides a converging power of about 40 dioptres, and the least converging power of the eye-lens behind the cornea is about 20 dioptres. From this rough data estimate the range of accommodation (i.e., the range of converging power of the eye-lens) of a normal eye.
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A man wearing glasses of focal length +1 m cannot clearly see beyond 1 m
Mark the correct options.
(a) If the far point goes ahead, the power of the divergent lens should be reduced.
(b) If the near point goes ahead, the power of the convergent lens should be reduced.
(c) If the far point is 1 m away from the eye, divergent lens should be used.
(d) If the near point is 1 m away from the eye, divergent lens should be used.
A person looks at different trees in an open space with the following details. Arrange the trees in decreasing order of their apparent sizes.
| Tree | Height(m) | Distance from the eye(m) |
| A | 2.0 | 50 |
| B | 2.5 | 80 |
| C | 1.8 | 70 |
| D | 2.8 | 100 |
Can virtual image be formed on the retina in a seeing process?
A normal eye has retina 2 cm behind the eye-lens. What is the power of the eye-lens when the eye is (a) fully relaxed, (b) most strained?
A myopic adult has a far point at 0.1 m. His power of accomodation is 4 diopters.
- What power lenses are required to see distant objects?
- What is his near point without glasses?
- What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be 2 cm.)
